THALATTOSAURIA
Millions of years before the first appearance of the mosasaurs in
geological history, another group of reptiles showing many curious
resemblances to them attempted a rather precarious existence in the
water. Its members survived long enough to acquire many structural
adaptations to a water life, long enough to become diversely modified,
but not long enough, apparently, to wander far from their birthplace,
not long enough to attain that security from their enemies and more
ambitious competitors, the early ichthyosaurs and plesiosaurs, to
insure them a long existence. They were only a partial success as water
reptiles.
It has been only within a few years that we have had any knowledge
whatever of them, and that knowledge is still very incomplete, too
incomplete to justify any attempt to picture them as living animals,
even though we take the liberties that some of our illustrators
of extinct animals feel warranted in assuming. The first known
specimens of these “sea-reptiles”—for that is the meaning of the word
Thalattosauria—were discovered and described by Professor J. C. Merriam
less than ten years ago, and all our knowledge of these animals is due
to the same author, who has studied attentively the known specimens,
all of which are preserved in the museum of the University of
California. The first discovered fragmentary specimens were confounded
with those of early ichthyosaurs, from the Upper Triassic rocks of
northern California with which they were associated. No specimen has
yet been found that is even approximately complete; some parts of the
skeleton are not yet known, even from fragmentary remains, and not till
other and more complete specimens have been found will it be possible
to determine the real form of the living animals or to decide what
their nearest relationships with other reptiles were. Professor Merriam
thinks that they were related most closely with the Rhynchocephalia
(p. 176) of which the _Sphenodon_, or tuatera, of New Zealand is the
only living representative, but whose direct genealogical history runs
back nearly or quite to the time in which the thalattosaurs lived. On
the other hand, there are so many resemblances to the mosasaurs shown
in the remains that have been discovered, that it is possible the
thalattosaurs were only a short-lived branch of the primitive lizards,
which we also know were in existence at the time when the thalattosaurs
lived. However, even though they resembled the mosasaurs, there could
have been no direct genealogical relationships between them, for it is
quite certain that the thalattosaurs very soon went out of existence,
leaving no descendants. But it matters little which were the land
forbears of the thalattosaurs; they present such distinct adaptations
to water life—characters all their own—that their ancestral kinship may
well be left to the future researches of the curious paleontologist.
For the present, at least, they may well be placed in an order
of reptiles all their own, as Professor Merriam has proposed—the
Thalattosauria.
[Illustration: FIG. 82.—Skull of _Thalattosaurus_. (After Merriam)]
No thalattosaurs were large animals. If they had the same proportions
between the lengths of head, body, and tail as the mosasaurs, none
exceeded seven feet in length, and they may have been even shorter,
though probably not much. The figure of the skull, as restored by
Professor Merriam, shows many striking aquatic adaptations, in the
elongated, pointed muzzle, in the large external nostrils, situated
far back toward the eyes, and in the well-ossified ring of bones
surrounding the eyeball. There is a parietal opening in the roof of
the skull, as in the modern lizards and tuatera; but it is not known
for certainty whether there were two openings on each side in the
roof of the skull, as in the modern tuatera. While this character may
seem trivial, it is really one of the most important in the reptilian
anatomy in determining the relationship and classification of reptiles.
The teeth are conical and pointed in the front end of the upper and
lower jaws, but farther back they are rounded, rugose, and obtuse, and
could have been used only for crushing hard objects, like mollusks,
crustaceans, etc. (Fig. 82). And not only was there a row of such teeth
on each jaw (only partly seen in the figure), but similar teeth covered
a large part of the palate. And the lower jaws, it is seen, are rather
massive.
The vertebrae were, of course, of the more primitive kind, that is,
with the ends concave, both in front and behind. It would have been
strange indeed were they of any other kind, since reptiles with
ball-and-socket joints to the vertebrae, that is, concave on one end
and convex on the other, as in nearly all living reptiles, did not come
into existence till long after the thalattosaurs had disappeared from
geological history; and it is also a curious fact that such vertebrae
appear to have originated only among animals crawling on land, so that
they would not have been a character acquired by the thalattosaurs
after descending into the water. It will be seen from the figure of
a dorsal vertebra that the rib was attached by a single articular
surface, almost exclusively to the body of the vertebra, quite like
those of all lizards, snakes, and mosasaurs, and unlike those of
other reptiles. This too may seem to be a trivial character to prove
relationships with the lizards, but it is a curious fact that no two
animals having different kinds of ribs are closely related to each
other. Possibly, however, this looser mode of attachment of the ribs
in the thalattosaurs was one of their peculiar adaptations to a water
life, and may not have been derived from their land ancestors.
[Illustration: FIG. 83.—Dorsal vertebra of _Thalattosaurus_. (After
Merriam.)]
[Illustration: FIG. 84.—_Thalattosaurus_: bones of front extremity:
_s_, scapula; _c_, coracoid; _h_, humerus; _r_, radius; _u_, ulna.
(After Merriam.)]
Of the limbs, only a few bones are known, but these are very
instructive. The arm bones, as shown in Fig. 84, are strikingly like
those of the mosasaurs, as will be seen by comparing the figure on p.
157. The humerus is a little more elongated than that of the mosasaurs,
more nearly like the mosasaurian femur. The shoulder-blade and the
coracoid are imperfectly ossified, as is seen from the figure—another
characteristic of aquatic life. What the fingers and toes were like
cannot be said; probably they were bound together by membrane, forming
swimming paddles similar to those of the mosasaurs. Some of the bones
referred to the pelvis are known, but it is not known whether they are
united to the spinal column by a sacrum, as in land animals. Nor is
anything certainly known of the hind leg or much of the tail. Since the
front legs show marked aquatic adaptations, it is altogether certain
that the hind legs will be found to be modified more or less, though
not so much modified as the front legs, because, as we have seen, the
front legs are always more specialized in aquatic animals than the
hind ones, even as the hind legs are more specialized than the front
ones in land animals. Possibly the hind legs will be found to be more
like those of the Thalattosuchia, as shown on p. 212, that is, partly
terrestrial in character. Doubtless the tail was long and flattened,
possibly with a terminal fin-like dilation, though this is less
probable.
As regards the habits and food of the thalattosaurs, no better summary
can be given than that of Professor Merriam, in his own words:
The remains of thalattosaurs are known only in purely marine
deposits containing little or no material of terrestrial
origin. They are associated with a fauna consisting of
numerous forms, both vertebrate and invertebrate, which
are not known to have existed away from marine areas. In
the structure of the skeleton we find the abbreviated and
broadened proximal segments of the limbs, the slender snout
with prehensile terminal teeth, and the median superior
nostrils, indicating a purely aquatic type. There can
scarcely be room for doubt that the thalattosaurs as a group
were typical marine forms. The larger and more specialized
species comprised in the genus _Thalattosaurus_ were
strictly natatory. They may have visited the shore, but,
like the plesiosaurs, were better fitted for swimming
than for crawling. Of the smaller _Nectosaurus_
we unfortunately do not know the limbs. They may have
been considerably less specialized, and the animal
to a correspondingly greater degree a shore-dweller.
_Nectosaurus_ is, however, found in the same deposits
with other forms and appears to be as common as the others;
so that it is safe to consider it as having passed the
greater part of its life away from the shore.
From what we know of the vertebral column of
_Thalattosaurus_ it appears that the animal had
a relatively short neck and a long dorsal region, the
proportions being nearly those in the vertebral column of
some mosasaurs. Only the anterior portion of the caudal
region is known. The slender, rounded neural spines with
well-developed articulating processes seen here are not
such as commonly appear in forms with a highly specialized
sculling tail, and it is hardly probable that a caudal fin
of large size was developed.
The anterior limbs evidently formed paddles of moderate
size. The posterior pair may have been larger, in
compensation for lack of a strong sculling tail. It is,
however, possible, that as in _Geosaurus_ (of the
thalattosuchian crocodiles) the hind limbs were not
typically natatory, and the distal end of the tail was
vertically expanded.
No specimens have yet been found which are well enough
preserved to show any remains of the stomach contents,
and we have no definite evidence concerning the food of
the thalattosaurs, more than is furnished by the general
structure of the animal. The character of the paddles, the
form of the skull, and the presence of slender prehensile
teeth in the terminal portions of the jaws would indicate
that they fed in part upon some swiftly moving prey which
was caught by a quick snap of the jaws, deglutition being
assisted by the curved teeth of the pterygoids. The heavy
vomerine and posterior mandibular teeth may have been used
for crushing the light shells of ammonites, which existed in
vast numbers in the same seas.